A pre-coated steel sheet has been broadly used in various fields such as a surfacing member for electric home appliance or an air conditioner and other goods, due to its good productivity compared with a steel sheet, which is formed to an objective shape prior to application of paint.
The pre-coated steel sheet is manufactured by applying synthetic resin paint to both surfaces or one surface (which will be an external surface of a product) of a steel sheet, and baking the applied paint to form a paint layer. A corrosion inhibitor is commonly added to the resin paint in order to improve corrosion resistance of the paint layer. Adhesiveness of the paint layer is enhanced by chemically converting the surface of a steel sheet to a chromated or phosphated state prior to paint application. The chemically converted surface layer also affects improvement of corrosion resistance.
In a process of manufacturing a painted steel sheet having a single coat, one kind of paint containing a color pigment and a corrosion inhibitor is applied to the surface and then baked. In a process of manufacturing a painted steel sheet having two or more coats, a primer containing an extender and a corrosion inhibitor is applied to a surface of the steel sheet, and then intercoat and topcoat paints, which do not contain a corrosion inhibitor, are applied to the undercoat. A back surface of the steel sheet may be also coated with paint containing a corrosion inhibitor.
Chromium compounds or pigments based on chromium compounds, e.g. zinc chromate, strontium chromate, red chromate and red silicochromate, have been used so far as a corrosion inhibitor, due to excellent corrosion-inhibiting faculty. However, there is a stronger demand in these days for provision of a steel sheet coated with a paint layer free from chromium compounds, which exert harmful influences on the environment. In order to cope with such the demand, a corrosion inhibitor prepared from porous silica particles, to which Ca, Zn, Co, Pb, Sr or Ba ion is bonded by ion-exchange, is proposed instead of chromium compounds.
The proposed corrosion inhibitor captures corrosive ions such as hydrogen ion and discharges the bonded ion in return. Especially, Ca ion-bonded corrosion inhibitor performs good corrosion-inhibiting faculty. Such corrosion inhibitor is ordinarily added to paint at a ratio of 2-50 parts by weight based on 100 parts by weight of resinous components in a paint layer. However, the Ca ion-bonded corrosion inhibitor is somewhat insufficient of corrosion and moisture resistance in comparison with chromium compounds, so that blisters often occur in a paint layer in a wet atmoshpere.